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Application of new technology liquid scintillation spectrometry to radiocarbon dating of tephra deposits, New Zealand
Quaternary International, Vol. 13/14,pp. 135-142, 1992. Printed in Great Britain. All rights reserved.
1040-6182/92 $15.00 © 1992 1NQUA/Pergamon Press Ltd
APPLICATION OF NEW TECHNOLOGY LIQUID SCINTILLATION SPECTROMETRY TO RADIOCARBON DATING OF TEPHRA DEPOSITS, NEW ZEALAND D.J. L o w e and A . G . H o g g Geochronology Research Unit, School of Science and Technology, University of Waikato, Private Bag 3105, Hamilton, New Zealand
Two major technological advances in the radiocarbon dating method have recently enhanced its potential application to tephra studies: the advent of accelerator mass spectrometry and the development of new technology liquid scintillation (LS) spectrometry. The new technology LS spectrometer represents a significant refinement of the conventional dating method based on liquid scintillation counting of benzene. It improves upon conventional LS counting by allowing spectral analysis, and by providing a high degree of counting stability and efficiency in an ultra low-level background radiation environment. These attributes enable radiocarbon dating with greater accuracy and statistical precision, and also allow the determination of both smaller and older samples than previously possible by conventional radiometric methods. The new Wallac Oy 'Quantulus' LS spectrometer has been in operation at the University of Waikato Radiocarbon Dating Laboratory since 1988. The instrument achieves ultra-low background levels by both passive and active forms of shielding. The shielding comprises a massive asymmetric passive lead shield surrounding an anticoincident liquid scintillation guard. In addition, the Quantulus contains dual multichannel analyzers which allow spectral analysis and 'windowless' data acquisition. The Quantulus LS spectrometers at the Waikato laboratory have been used to date a variety of carbonaceous materials associated with tephra deposits in New Zealand, ranging in age from ca. 0.1-55 ka. In particular, we have tested the capability of the Quantulus for determining ages of samples that are typically more difficult or impossible to date by conventional methods: (1) very young samples (e.g. Tarawera Tephra, erupted ca. 100 years ago); (2) older samples (e.g. Mangaone Tephra, erupted ca. 30 ka); and (3) samples containing only sparse carbon (e.g. lake sediments associated with tephra layers aged ca. 20 ka or less). The Quantul0s LS spectrometer is both more accurate and precise (statistical counting errors are reduced) than conventional instruments, is capable of extending the limits of detection at both ends of the age scale, and has a smaller sample handling ability. Analyses can be obtained at comparatively low cost. Such advances are potentially beneficial to tephrochronology and volcanology and a wide range of applications in Quaternary research.
INTRODUCTION Since its development in the late 1940s-early 1950s, the radiocarbon dating method has become central to the development of tephrostratigraphy and tephrochronology and their application to Quaternary research (e.g. Naeser et al., 1981; Lowe, 1990). Other dating techniques have subsequently been developed and applied to tephra layers but, for the majority of deposits of Late Pleistocene or Holocene age, the radiocarbon method remains the most widely used. In the past decade, however, there have been two major technological advances in the radiocarbon method that have enhanced its potential application to tephra studies (tephrology). The first of these advances is the advent of accelerator mass spectrometry (AMS). The AMS method, which measures individual 14C atoms present, requires only 1 mg or less of elemental carbon, and very short measurement times (Linick et al., 1989). However, AMS dating is expensive. The second advance has been the complementary development of low-level, higher-precision liquid scintillation (LS) spectrometry, which represents a major refinement of conventional radiometric [3-decay counting methods based on liquid scintillation counting of benzene (Polach, 1987a, b, 1989). In the first part of this paper, we outline the way in which the new technology LS spectrometers have
improved upon the conventional scintillation counting method, and the consequent advantages they provide for radiocarbon dating. We then examine a number of radiocarbon dates from a variety of tephra deposits in New Zealand as an initial test of the LS spectrometers' capability based on analyses carried out at the University of Waikato Radiocarbon Dating Laboratory, Hamilton, New Zealand. We wish to emphasize that the standard errors reported here are based on statistical counting errors alone and do not incorporate other sources of error associated with the radiocarbon dating method. RADIOCARBON DATING BY LIQUID SCINTILLATION COUNTING In this method, the carbon in a sample is converted to benzene via a series of well-established chemical steps (e.g. Polach, 1987b; Hogg et al., 1987). Betaradiation emitted by the decay of the 14C isotope in the benzene solvent reacts with a scintillation solute of butyl-PBD to release bursts of light, which are recorded (counted) by photomultiplier tubes over a period of time. Because the natural levels of 14C in the Earth's atmosphere are very low (about 1 × 10-1°%; Linick et al., 1989), accurate 14C measurements require extremely high precision. Such precision is not easily achieved because of the influence of cosmic and
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D.J. Lowe and A.G. Hogg
environmental background radiation, radiation from other isotopes that may be present, electronic noise, and other factors (Polach, 1987b). These background factors limit the accuracy, precision, and range of the radiocarbon dating method because a meaningful age can only be obtained if the sample activity is at least three standard deviations above background activity (Gupta and Polach, 1985). Consequently, each of the components of the LS counting system needs to be totally optimized for low-level counting so as to maintain high efficiency whilst significantly reducing the background (Polach et al., 1984, 1988; Polach, FIG. 1. Passive and active shielding of the Quantulus LS spectrometer. PMT, photomultiplier tube; S, sample counting vial with 1987b). Cadmium shielding is interlayered The new technology LS spectrometers improve upon copper top; L, liquid scintillator. with the copper. conventional scintillation counting by effectively reducing this background activity to ultra-low levels, and by providing a greater degree of counting stability and 14C Counting Vials detection efficiency. In addition, the sample isotope Vials need to have a high photon transmission and background activities are not simply counted efficiency, a low self-induced radiation component, and between fixed windows but are displayed as 'window- exhibit no memory. The most commonly used vials less' energy spectra using two computer-controlled include those made from borosilicate glass (4°K a n d multi-channel analyzers (MCAs) to monitor, evaluate, 226Ra free i.e. 'low-K' glass), fused silica (quartz), and and validate data and instrument performance (Polach polytetrafluoroethylene (TeflonTM). Recent research et al., 1983a, 1984). at the Waikato Radiocarbon Dating Laboratory has In the next section we summarize how some of these resulted in the development of a new vial made of highimprovements have been achieved. More detailed purity synthetic silica (Hogg et al., 1991). The counting discussions are reviewed by Polach (1987a, b, 1989). characteristics of the new silica vial are compared with We refer specifically to the new technology LS spectro- those of Teflon TM and low-K glass in Table 1. The first meter named the 'Quantulus' that was developed and two columns show background levels and counting assessed by Wallac Oy (Turku, Finland) and Radiocar- efficiencies; the second two columns show the oldest bon Dating Research, Australian National University and youngest ages of samples obtainable using 3 ml (ANU) (Canberra) (Polach et al., 1984, 1988). The first samples of benzene. The results demonstrate that the of three Quantulus LS spectrometers was acquired by synthetic silica vials are as good as those made from the University of Waikato Radiocarbon Dating Labor- TeflonTM, and superior to low-K glass vials, in peratory in 1988. formance. In addition, however, Hogg et al. (1991) found the synthetic silica vials to be the best in terms THE QUANTULUS LIQUID SCINTILLATION of physical and handling properties, making them SPECTROMETER superior even to TeflonTM. Background Reduction by Shielding The Quantulus LS spectrometer achieves ultra-low background levels by both passive and active forms of shielding. The passive shielding consists of a 650 kg asymmetric lead block that encloses the sample photomultiplier tubes (PMTs), and attenuates cosmic and environmental radiation. The shield is constructed from low residual activity lead (from Boliden Mine, Sweden) and the internal cavity surrounding the PMTs is lined with high purity copper (Fig. 1). A layer of cadmium, lined with copper, surrounds the sample PMTs to shield against neutrons (Kojola et al., 1984). Within the lead shield are two sets of PMTs. The lower pair of tubes counts activity in the sample vial, which is hoisted into position. These tubes are totally enclosed by an anti-coincident guard containing a liquid scintillator (Fig. 1). The active, or electronic, shielding consists of the upper pair of PMTs attached to this guard. These tubes can detect and thus reject any pulses caused by cosmic radiation components (such as muons) rather than sample activity (Polach, 1987b).
Data Processing by Spectral Analysis Dual MCAs on the Quantulus allow counting data and instrument performance to be continuously monitored using 'windowless' counting (Polach et al., 1983a). The spectra in Fig. 2 provide an example. They are shown as the logarithm of relative pulse height (energy) of t4c activity, given in terms of MCA channel
TABLE 1, Quantulus counting characteristics* for high-purity synthetic silica 3 ml vials compared with those made from teflon T M and low-K glass Vial material Silica TeflonT M Low-K glass
Background (cpm)
Efficiency (%)
t,,axt (years)
tm~. (years)
0.32 0.29 0.50
83.3 83.7 69.2
55,000 55,400 51,700
40 39 44
*After Hogg et al. (1991). CThe maximum attainable age would increase using vials of larger volume.
New TechnologyLiquid Scintillation Spectrometry
137
more or less evenly around a mean, as they are in Fig. 3.
Electronics and Software The final technical aspect to consider is that the O Quantulus LS spectrometer has been designed with state-of-the-art electronics (Polach, 1987b) that include e.. constant monitoring of the high voltage supply to the PMTs. This is an important development because it provides an extremely stable counting environment, thus improving the accuracy of the instrument for 14C dating. Other electronic controls include a coincidence 26o 560 700 resolution time of 20 nsec, two-position coincidence MCA channel numbers bias, and pulse amplitude comparison circuitory (e.g. FIG. 2. MCA spectra for 1~C sample (S), modern (secondary) see Polach et al., 1983b). Their functions are briefly standard (M), and background(B). Dashed lines L and R flankthe computer-selected 'soft window' best suited for ~4C dating this outlined below. sample (WK-1533a) (see text). (i) Coincidence resolution time. All LS counters using two 180° opposed PMTs record pulses initiated within each PMT only if they fall within a set numbers (0-250 keV) on the horizontal axis, against coincidence resolution time interval. This reduces the counts per channel on the vertical axis. Curve S is that possibility of pulses generated within either of the of the 'unknown' sample being dated; curve M repre- PMTs being recorded as genuine decay events, and sents the higher-activity modern standard; and curve B results in a lowering of background levels. The Quantuis the background activity. In terms of count rates for 5 lus has a very short coincidence time (20 nsec) ml benzene samples, the modern secondary standard compared with the average of 25-40 nsec in conven(ANU Sucrose) is registering 74 counts per minute tional counters. (ii) Two position coincidence bias. In most LS (cpm) (equivalent to 0.95 Oxalic Acid Standard count counters, pulses generated within each PMT are sumrate of 47 cpm), the unknown sample 46 cpm, and the med so long as they occur within the specified coincibackground just 0.4 cpm. Old-technology counters dence resolution time. The Quantulus incorporates a have background count rates of 3-4 cpm, so the two position, factory set coincidence bias threshold that Quantulus backgrounds are around ten times lower the summed pulses must exceed to be recognized as (Polach et al., 1988). true decay events. The coincidence bias threshold is The dashed vertical lines flanking the spectra in Fig. necessary to reduce background due to optical cross2 mark the region of interest or 'soft window' selected talk. Many conventional LS counters utilize a single for counting this particular sample. This window can be coincidence bias level which is optimized for 3H adjusted (unlike old-technology counters in which the window is fixed) to optimize the counting efficiency counting. The two position (HI-LO) switch on the (Polach et al., 1983a). Radioactivity from isotopes Quantulus incorporates coincidence bias levels optiother than 14C, such as radon (222Rn), generate extra mised for both 3H and 14C counting. (iii) Pulse amplitude comparison (PAC). The PAC counts which, if undetected, result in erroneous 14C age determinations (Polach and Kaihola, 1988). The spectral analysis display facility allows such spurious activity to be recognized and hence its influence deleted or 40" corrected. (Radon is generated from traces of uranium 30" and radium present within the sample and can be incorporated into the benzene during synthesis.) 2ff The sample shown in Fig. 2 has an age calculated at o o o 10" around 200 + 20 years after 5000 min (83 hr) of o o o counting time. Counting statistics for this sample (Fig. Mean 3) show how the count rate has varied over this 5000 O min counting period, with each of the points represent-10" ing 500 min of counting time. The distribution of the -20" points therefore provides a measure of stability of the counter - - an unstable counter would show much wider -30" increases or decreases from point to point than the -4~7 pattern shown here. The plot may also show whether Time any short-lived isotope is present as a contaminant in the sample. Thus the data points would tend to lie FIG. 3. Statistical plot of count rate variations over ten 500 min along a negative slope rather than being distributed counting periods. Data are for sample in Fig. 2.
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D.J. Lowe and A.G. Hogg
circuitory compares the amplitude of the pulses from each PMT and, if the pulse heights differ by more than a selected amount, the pulses are rejected as background events. PAC levels are software selectable, and determine the amount of pulse amplitude variation that may be tolerated. The optimum PAC setting is experimentally determined, and results in lower background levels without significant reduction in counting efficiency. Additional software features of the Quantulus include low-level statistical count rate evaluation and validation parameters as well as ~4C age calculations based on the 'windowless' or 'soft window' spectral analysis (Polach, 1989). The spectral analysis facility further improves accuracy through detection of any radioactive contaminants. ADVANTAGES ACHIEVED BY THE QUANTULUS LS SPECTROMETER AND IMPORTANCE TO RADIOCARBON DATING
The advances achieved by the technological improvements described in the previous section arise primarily from the Quantulus' extreme counting stability, its MCA spectral analysis facility, and from the ultra-low backgrounds which reduce standard counting errors. In terms of radiocarbon dating, these advances are advantageous in four main ways. (1) The accuracy of dating is improved. (2) The precision of ages is increased. The best precision achievable by the Quantulus (___20--25 years*) is higher than that of the AMS system (Linick et al., 1989; Gillespie, 1991), and is illustrated in the applications section below. (3) Much smaller samples are required. Although LS spectrometers such as the Quantulus are now capable of dating samples containing 100 mg or less of carbon (Polach, 1987b; minimum amounts depend on sample age), the AMS system is clearly superior in its ability to handle samples with less than 100 mg of carbon (Linick et al., 1989). (4) Older samples can be dated. The maximum theoretical age obtainable using the Quantulus is around 65 ka (Polach, 1987a). Ages exceeding ca. 45 ka generally require large samples which must be unusually well preserved. The previous instrumental age limit using conventional counters was usually in the range 30-40 ka, with AMS having a similar limit (Linick et al., 1989; Gillespie, 1991). APPLICATION TO TEPHRA DEPOSITS IN NEW ZEALAND
In this section of the paper we describe four applications of the Quantulus LS spectrometer to the dating of Late Quaternary tephra deposits in North Island, New Zealand. Three examples relate to tephras *For 5 ml vials of benzene counted for 5000 min.
erupted from the Okataina Volcanic Centre in the Taupo Volcanic Zone (Fig. 4; Froggatt and Lowe, 1990); the fourth example deals with dates on a sequence of tephra layers interbedded with late glacialpostglacial lake sediments. Tarawera Tephra Mt. Tarawera (Fig. 4) last erupted on 10 June, 1886. The eruption was a plinian basaltic event that produced the Tarawera Tephra Formation, devastating much of the surrounding landscape and several villages (Walker et al., 1984; Froggatt and Lowe, 1990). The forest on the slopes of the mountain was killed by the fallout of hot scoria but a number of mature totara trees (Podocarpus hallii) were left standing on the upper slopes (R.F. Keam, pers. commun., 1989) and are still to be found today scattered through the regenerating forest. We estimate that the trees were probably around 200 years old at the time of their death in 1886. They have begun to rot and the outermost wood is commonly soft or has evidently sloughed off. We sampled a branch from one of these trees and dated the most sound outermost wood layers which were extracted with a drill to avoid the obvious borings (Fig. 5a). The four ages obtained (Table 2) range from 180 to 240 BP with standard counting errors of + 30 years. (These and the other ages reported in the text are conventional radiocarbon ages based on the old halflife + 1 standard deviation: Hogg et al., 1987.)The ages on cellulose are likely to be the most reliable because this fraction is the least susceptible to contamination (Gupta and Polach, 1985). The error-weighted pooled mean (Gupta and Polach, 1985) of the four ages is 210 + 15 BP. Recent interlaboratory studies have emphasized the need to use error multipliers in calculating calibrated (calendar) ages using tree ring chronologies because quoted errors may inadequately describe the true variability (e.g. Scott et al., 1990). After applying the 30 year southern hemisphere correction to Stuiver and Pear-
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FIG. 4. Locations of Tarawera and Haroharo volcanoes within the Okataina Volcanic Centre (VC), North Island, and the tephra sites studied: 1, trees killed by fall of Tarawera Tephra (grid reference* V16/194232); 2, Kaharoa Tephra section (V16/175197); 3, Mangaone Tephra section (W16/730378); 4, Lake Maratoto (S15/130663). TVZ = Taupo Volcanic Zone. *Based on the 1:50,000 topographical map series NZMS260.
New Technology Liquid Scintillation Spectrometry
FIG. 5. (a) (upper) Slice from branch of totara tree (Podocarpus hallii) killed by fall of Tarawera Tephra in 1886 (sample WK1534). (Photo: R. Clayton.) (b) (lower) Carbonized logs (Podocarpus hallii) within ignimbrite deposited during Kaharoa eruptive episode. Sample WK-1532 was on bark from the log on the left. Lens cap 5 cm in diameter. (Photo: D.J. Lowe.)
139
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D.J. Lowe and A.G. Hogg
TABLE 2. Radiocarbon ages associated with the tephra deposits examined in this study* and obtained by the Quantulus LS spectrometer Age (BP ± 1 sd:~) Waikato lab no.
Old T1/2
New T1/2
~13C (%o)
Sampler
Tarawera Tephra WK-1533a WK-1533b WK-1534a WK-1534b
210 240 180 210
± ± ± ±
30 30 30 30
210 ± 240± 190± 220 ±
30 30 30 30
-25.2 -25.4 -25.0 -24.0
W CL(1) W CL(2)
620 680 700 660
± ± ± ±
30 35 30 45
640 700 720 680
35 35 30 45
-27.5 -24.7 -24.9 -25.5
CW CW CT CW
-29.1 -29.8
P P
Kaharoa Tephra WK-1531 WK-1532 WK-1535 WK-1346
+ + + +
Mangaone Tephra WK-1525 WK-1526
31,600± 250 31,700± 250
32,500 + 250 32,700 + 250
*Site locations given in Fig. 4. t W = wood; CL(1) = cellulose extracted from WK-1533a; CL(2) = cellulose extracted from WK-1534a; CW = carbonized wood; CT = carbonized twigs; P = Peat. ~sd = standard deviation.
son's (1986) calibration curves, we consequently used an arbitrary theoretical error multiplier of 2.0 to determine the calibrated age for the Tarawera wood. Using 95% confidence limits (two sigma errors), the curves show that multiple calibrated ages are possible, the first having the greatest probability: AD 1722-1820 (58%), AD 1653--1698 (20%), AD 1920-1955 (20%), and AD 1852-1866 (2%). None of these calibrated age ranges encompasses the known year of the Tarawera eruption in AD 1886 but, taking the age of the tree and its condition into account, the most likely age range is still reasonably close. Notwithstanding this, it is evident that Quantulus is capable of precisely dating material as young as 100-200 years. We are currently looking to date more suitable carbonaceous material associated with the emplacement of Tarawera Tephra at other sites. In the interim, results from the Glasgow Intercomparison of Radiocarbon Laboratories Study help to demonstrate the accuracy of the Quantulus for dating very young materials, as attempted for the Tarawera Tephra. Glasgow intercalibration sample 'Wood-3' was dated via dendrochronology at AD 1841-1870 which equates to an expected conventional radiocarbon age of about 100 BP (Aitchison et al., 1990). We obtained a statistically identical age of (WK~cs-1340) 85 + 31 BP for 'Wood-3' using the Quantulus at the Waikato Laboratory.
Kaharoa Tephra The Kaharoa eruptive episode preceded the Tarawera eruption on Mt. Tarawera (Fig. 4). This eruption was rhyolitic and produced airfall tephra deposits, pyroclastic flows and surges, and block-and-ash flows, all of which make up the Kaharoa Tephra Formation (Froggatt and Lowe, 1990). For some time there has
been a question about the age of the eruption, because 14C ages obtained seem to cluster into two groups, one around 900 and the other 700 BP, although there is currently no stratigraphic evidence for an hiatus during the eruptive episode (Froggatt and Lowe, 1990). We obtained four new ages from a range of carbonised materials including twigs, small branches, and bark from small logs (Fig. 5b) in ignimbrite and block-andash flow deposits in a section at the foot of the mountain (Fig. 4). The ages obtained (Table 2) range from 620 to 700 BP with standard counting errors of 30--45 years, and have a pooled mean of 665 + 17 BP. These results thus confirm that the bulk of the Kaharoa deposits were erupted around, or soon after, ca. 700 BP. However, it remains uncertain if a smaller precursory eruptive event took place because these younger ages differ significantly from those of ca. 900 BP. New stratigraphic work and further dating using the Quantulus may resolve this uncertainty.
Mangaone Tephra The Mangaone Subgroup is a group of eight tephra formations of Late Pleistocene age erupted from the Haroharo volcano (Fig. 4; Howorth, 1975). The three youngest formations are, respectively from stratigraphic bottom to top, the Mangaone, Awakeri, and Omataroa tephras. Mangaone Tephra has a pooled mean age (n = 10) of 27,730 + 350 BP; Awakeri Tephra is not dated; and Omataroa Tephra has a pooled mean age (n = 3) of 28,220 + 630 BP (Froggatt and Lowe, 1990). The mean ages for the Omataroa and the Mangaone Tephras are not significantly different yet the Mangaone Tephra must be older than both the Awakeri and Omataroa tephras because it underlies them. Furthermore, the presence of a weak paleosol on the Mangaone Tephra (Howorth, 1975) indicates that some time elapsed before it was buried by the Awakeri and Omataroa deposits. If all the ages on Omataroa Tephra are accepted as valid, then it is evident that some of the dates obtained on the Mangaone Tephra are underestimates. Consequently, Froggatt and Lowe (1990) suggested that Mangaone Tephra was erupted perhaps 30,000 years ago. In order to test this suggestion, we sampled a section at Nukuhou (Site 3 in Fig. 4) containing some Mangaone Subgroup tephra formations interbedded with peat deposits. The section was studied previously by McGlone et al. (1984) who identified the tephra layers present. We confirmed most of the correlations through mineralogy and electron microprobe analysis of glass and dated two 5 cm thick layers of peat at the base of Mangaone Tephra in the upper part of the section. Ages of 31,600 and 31,700 BP, both with standard counting errors of + 250 years (Table 2), were obtained. These results thus support an age of ca. 30,000-32,000 years for the eruption of Mangaone Tephra. In addition, we would like to contrast the relatively low counting errors on these ages with those associated
141
New Technology Liquid Scintillation Spectrometry
with the ages obtained previously on Mangaone Tephra at other locations where the Quantulus was not used. For ten age determinations, these errors range from 400 to 2200 years, but almost all exceed 800 years (see Froggatt and Lowe, 1990), thus demonstrating the improved precision obtainable using the Quantulus LS spectrometer. There are few ages on tephra deposits in New Zealand older than ca. 30 ka (Froggatt and Lowe, 1990) and we are currently attempting to obtain carbonaceous material associated with such undated or poorly dated tephra formations. The oldest ages so far assayed in the Waikato Radiocarbon Dating Laboratory using the Quantulus are > 55,000 years.
unable to date some samples from these lake sediments with low carbon contents because sample activities were statistically indistinguishable from background activities - - the background count rates were too great - - but no such restrictions have occurred using the Quantulus. CONCLUSIONS
New technology LS spectrometry provides both more accurate and more precise dates than conventional instruments, can date much smaller samples than previously possible with radiometric systems, and is capable of extending the limits of detection at both ends of the age scale. These attributes have been Tephra Layers in Lake Sediments achieved in the Quantulus LS spectrometer by providThis final application further emphasizes the value of ing a high degree of counting stability and efficiency in the low background levels attained using the Quantuan ultra low-level background radiation environment lus. Some years ago, Green and Lowe (1985) and Lowe brought about by minimizing all known factors con(1988) took a number of cores from lakes formed ca. tributing to background, and by data evaluation and 17,000 BP near Hamilton in the North Island, including validation through a computer-controlled spectral Lake Maratoto (Site 4 in Fig. 4), as part of a wider analysis facility (Polach, 1987b). The AMS system, pale•environmental project. A large number of the which should be regarded as complementary to LS thin, multiple tephra layers contained within the peaty spectrometry (Polach, 1987a), is capable of dating the sediments were dated using old-style LS counters smallest of samples in a very short measurement time (Hogg et al., 1987). Figure 6 shows a plot of the ages but is less precise and considerably more costly than LS from Lake Maratoto recalculated as if they had been spectrometry. counted using the Quantulus LS spectrometer (normalThe advances provided by the new technology LS ized to 3000 min count time). The horizontal scale gives spectrometry are potentially of great benefit to net count rates from the old LS counter while the tephrochronology and volcanology and to a wide range vertical axis shows differences in standard error beof multidisciplinary applications in Quaternary retween the old LS counter and the Quantulus, expressed search. Four examples relating to tephra deposits in as percentage decrease i.e. (old counter error New Zealand (Tarawera, Kaharoa, and Mangaone Quantulus error/Quantulus error) x 100. For example, tephra formations, and Late Quaternary tephras in lake sample WK-240 was originally dated at 16,900 + 450 sediments) help show these benefits. BP but the equivalent Quantulus date would be 16,900 +_ 135 BP, a reduction in the standard counting error of nearly 250%. ACKNOWLEDGEMENTS Thus, the Quantulus gives greater precision which is of increasing importance as sample activity diminishes We thank Henry Polach and colleagues at the ANU Radiocarbon towards the left hand end of the plot. Furthermore, we Dating Research Laboratory for valued assistance in the operation of have in the past (using old style LS counters) been the Waikato Radiocarbon Dating Laboratory. The continuing support of technical staff at the Waikato Laboratory is also gratefully acknowledged. The University Grants Committee and the University of Waikato Research Committee (UWRC) provided funding towards purchase of two of the Quantulus spectrometers. The UWRC also provided funding for some of the dates reported in this paper. We thank Paul Froggatt (Victoria University of Wellington) for undertaking electron microprobe analyses of tephra samples from the Nukuhou section, Peter de Lange (Department of Conservation) for identifying carbonized wood samples, Ron Keam (University of Auckland) who told us about the in situ trees on Mt. Tarawera killed by the eruption, and an anonymous reviewer who provided useful comments.
250 • WK-240
~ 2oog t50e~ 100-
REFERENCES 50
0
I 2
I 4
I 6
I 8
10
Net count rate (cpm - old counter)
FIG. 6. Normalized net count rates for lake sediment ages (obtained using old-style LS counters, and recalculated as if they had been counted by Quantulus) plotted against differences in standard error expressed as percentage decrease (see text).
Aitchison, T.C., Scott, E.M., Harkness, D.D., Baxter, M.S. and Cook, G.T. (1990). Report on Stage 3 of the intercalibrative programme. Radiocarbon, 32, 271-278. Froggatt, P.C. and Lowe, D.J. (1990). A review of late Quaternary silicic and some other tephra formations from New Zealand: their stratigraphy, nomenclature, distribution, volume, and age. New Zealand Journal o f Geology and Geophysics, 33, 89-109.
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Gillespie, R. (1991). The limits of radiocarbon technology. In: Gillespie, R. (ed.), Quaternary Dating Workshop, pp. 8-10. Dept. of Biogeography and Geomorphology, Australian National University, Canberra. Green, J.D. and Lowe, D.J. (1985). Stratigraphy and development of ca. 17,000 year old Lake Maratoto, North Island, New Zealand, with some inferences about postglacial climatic change. New Zealand Journal of Geology and Geophysics, 28, 675-699. Gupta, S.K. and Polach, H.A. (1985). Radiocarbon Dating Practices at ANU. Handbook, Radiocarbon Dating Research, Australian National University, Canberra. Hogg, A.G., Lowe, D.J. and Hendy, C,H. (1987). University of Waikato radiocarbon dates 1. Radiocarbon, 29, 263-301. Hogg, A.G., Polach, H.A., Robertson, S. and Noakes, J. (1991). Application of high purity synthetic quartz vials to liquid scintillation low-level ~4C counting of benzene. In: Ross, H., Noakes, J. and Spaulding, J. (eds), Liquid Scintillation Counting and Organic Scintillators. Lewis Publishers. Howorth, R. (1975). New formations of late Pleistocene tephras from the Okataina Volcanic Centre, New Zealand. New Zealand Journal of Geology and Geophysics, 18, 683-712. Kojola, H., Polach, H., Nurmi, J., Oikari, T. and Soini, E. (1984). High resolution low-level liquid scintillation 13-spectrometer. International Journal of Applied Radiation Isotopes, 35, 949-952. Linick, T.W., Damon, P.E., Donahue, D.J. and Jull, A.J.T. (1989). Accelerator mass spectrometry: the new revolution in radiocarbon dating. Quaternary International, 1, 1-6. Lowe, D.J. (1988). Stratigraphy, age, composition, and correlation of late Quaternary tephras interbedded with organic sediments in Waikato lakes, North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 31, 125-165. Lowe, D.J. (1990). Tephra studies in New Zealand: an historical review. Journal of the Royal Society of New Zealand, 20, 119-150. McGlone, M.S., Howorth, R. and Pullar, W.A. (1984). Late Pleistocene stratigraphy, vegetation and climate of the Bay of Plenty and Gisborne regions, New Zealand. New Zealand Journal of Geology and Geophysics, 27. 327-350. Naeser, C.W., Briggs, N.D., Obradovich, J.D. and Izett, G.A.
(1981). Geochronology of Quaternary deposits. In: Self, S. and Sparks, R.S.J. (eds), Tephra Studies, pp. 13-47. D. Reidel, Dordrecht. Polach, H.A. (1987a). Perspectives in radiocarbon dating by radiometry. Nuclear Instruments and Methods, B29, 415-423. Polach, H.A. (1987b). Evaluation and status of liquid scintillation counting for radiocarbon dating. Radiocarbon, 29, 1-11. Polach, H.A. (1989). Liquid scintillation ~4C spectrometry: errors and assurances. Radiocarbon, 31, 327-331. Polach, H.A. and Kaihola, L. (1988). Determination of radon by liquid scintillation ~13 particle spectrometry: towards the resolution of a ~4C dating problem. Radiocarbon, 30, 19-24. Polach, H.A., Robertson, S., Butterfield, D., Gower, J. and Soini, E. (1983a). The 'windowless' approach to scintillation counting: low-level C-14 as an example. In: McQuarrie, S.A., Ediss, C. and Wiebe, L.I. (eds), Advances in Scintillation Counting, pp. 494-507. University of Alberta Press, Edmonton. Polach, H.A., Nurmi, J., Kojola, H. and Soini, E. (1983b). Electronic optimization of scintillation counters for detection of low-level 3H and 14C. In: McQuarrie, S.A., Ediss, C. and Wiebe, L.I. (eds), Advances in Scintillation Counting, pp. 420-441. University of Alberta Press, Edmonton. Polach, H.A., Kojola, H., Nurmi, J. and Soini. E. (1984). Multiparameter liquid scintillation spectrometry. Nuclear Instruments and Methods, BS, 439-442. Polach, H.A., Calf, G., Harkness, D., Hogg, A.G., Kaihola, L. and Robertson, S. (1988). Performance of new technology liquid scintillation counters for ~4C dating. Nuclear Geophysics, 2, 75-79. Scott, E.M., Baxter, M.S., Aitchison, T.C., Harkness, D.D. and Cook, G.T. (1990). An overview of some interlaboratory studies. Radiocarbon, 32, 259-265. Stuiver, M. and Pearson, G.W. (1986). High-precision calibration of the radiocarbon time scale, AD 1950-500 BC. Radiocarbon, 28, 805-838. Walker, G.P.L., Self, S. and Wilson, L. (1984). Tarawera 1886, New Zealand - - a basaltic plinian eruption. Journal of Volcanology and Geothermal Research, 21, 61-78.
1040-6182/92 $15.00 © 1992 1NQUA/Pergamon Press Ltd
APPLICATION OF NEW TECHNOLOGY LIQUID SCINTILLATION SPECTROMETRY TO RADIOCARBON DATING OF TEPHRA DEPOSITS, NEW ZEALAND D.J. L o w e and A . G . H o g g Geochronology Research Unit, School of Science and Technology, University of Waikato, Private Bag 3105, Hamilton, New Zealand
Two major technological advances in the radiocarbon dating method have recently enhanced its potential application to tephra studies: the advent of accelerator mass spectrometry and the development of new technology liquid scintillation (LS) spectrometry. The new technology LS spectrometer represents a significant refinement of the conventional dating method based on liquid scintillation counting of benzene. It improves upon conventional LS counting by allowing spectral analysis, and by providing a high degree of counting stability and efficiency in an ultra low-level background radiation environment. These attributes enable radiocarbon dating with greater accuracy and statistical precision, and also allow the determination of both smaller and older samples than previously possible by conventional radiometric methods. The new Wallac Oy 'Quantulus' LS spectrometer has been in operation at the University of Waikato Radiocarbon Dating Laboratory since 1988. The instrument achieves ultra-low background levels by both passive and active forms of shielding. The shielding comprises a massive asymmetric passive lead shield surrounding an anticoincident liquid scintillation guard. In addition, the Quantulus contains dual multichannel analyzers which allow spectral analysis and 'windowless' data acquisition. The Quantulus LS spectrometers at the Waikato laboratory have been used to date a variety of carbonaceous materials associated with tephra deposits in New Zealand, ranging in age from ca. 0.1-55 ka. In particular, we have tested the capability of the Quantulus for determining ages of samples that are typically more difficult or impossible to date by conventional methods: (1) very young samples (e.g. Tarawera Tephra, erupted ca. 100 years ago); (2) older samples (e.g. Mangaone Tephra, erupted ca. 30 ka); and (3) samples containing only sparse carbon (e.g. lake sediments associated with tephra layers aged ca. 20 ka or less). The Quantul0s LS spectrometer is both more accurate and precise (statistical counting errors are reduced) than conventional instruments, is capable of extending the limits of detection at both ends of the age scale, and has a smaller sample handling ability. Analyses can be obtained at comparatively low cost. Such advances are potentially beneficial to tephrochronology and volcanology and a wide range of applications in Quaternary research.
INTRODUCTION Since its development in the late 1940s-early 1950s, the radiocarbon dating method has become central to the development of tephrostratigraphy and tephrochronology and their application to Quaternary research (e.g. Naeser et al., 1981; Lowe, 1990). Other dating techniques have subsequently been developed and applied to tephra layers but, for the majority of deposits of Late Pleistocene or Holocene age, the radiocarbon method remains the most widely used. In the past decade, however, there have been two major technological advances in the radiocarbon method that have enhanced its potential application to tephra studies (tephrology). The first of these advances is the advent of accelerator mass spectrometry (AMS). The AMS method, which measures individual 14C atoms present, requires only 1 mg or less of elemental carbon, and very short measurement times (Linick et al., 1989). However, AMS dating is expensive. The second advance has been the complementary development of low-level, higher-precision liquid scintillation (LS) spectrometry, which represents a major refinement of conventional radiometric [3-decay counting methods based on liquid scintillation counting of benzene (Polach, 1987a, b, 1989). In the first part of this paper, we outline the way in which the new technology LS spectrometers have
improved upon the conventional scintillation counting method, and the consequent advantages they provide for radiocarbon dating. We then examine a number of radiocarbon dates from a variety of tephra deposits in New Zealand as an initial test of the LS spectrometers' capability based on analyses carried out at the University of Waikato Radiocarbon Dating Laboratory, Hamilton, New Zealand. We wish to emphasize that the standard errors reported here are based on statistical counting errors alone and do not incorporate other sources of error associated with the radiocarbon dating method. RADIOCARBON DATING BY LIQUID SCINTILLATION COUNTING In this method, the carbon in a sample is converted to benzene via a series of well-established chemical steps (e.g. Polach, 1987b; Hogg et al., 1987). Betaradiation emitted by the decay of the 14C isotope in the benzene solvent reacts with a scintillation solute of butyl-PBD to release bursts of light, which are recorded (counted) by photomultiplier tubes over a period of time. Because the natural levels of 14C in the Earth's atmosphere are very low (about 1 × 10-1°%; Linick et al., 1989), accurate 14C measurements require extremely high precision. Such precision is not easily achieved because of the influence of cosmic and
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D.J. Lowe and A.G. Hogg
environmental background radiation, radiation from other isotopes that may be present, electronic noise, and other factors (Polach, 1987b). These background factors limit the accuracy, precision, and range of the radiocarbon dating method because a meaningful age can only be obtained if the sample activity is at least three standard deviations above background activity (Gupta and Polach, 1985). Consequently, each of the components of the LS counting system needs to be totally optimized for low-level counting so as to maintain high efficiency whilst significantly reducing the background (Polach et al., 1984, 1988; Polach, FIG. 1. Passive and active shielding of the Quantulus LS spectrometer. PMT, photomultiplier tube; S, sample counting vial with 1987b). Cadmium shielding is interlayered The new technology LS spectrometers improve upon copper top; L, liquid scintillator. with the copper. conventional scintillation counting by effectively reducing this background activity to ultra-low levels, and by providing a greater degree of counting stability and 14C Counting Vials detection efficiency. In addition, the sample isotope Vials need to have a high photon transmission and background activities are not simply counted efficiency, a low self-induced radiation component, and between fixed windows but are displayed as 'window- exhibit no memory. The most commonly used vials less' energy spectra using two computer-controlled include those made from borosilicate glass (4°K a n d multi-channel analyzers (MCAs) to monitor, evaluate, 226Ra free i.e. 'low-K' glass), fused silica (quartz), and and validate data and instrument performance (Polach polytetrafluoroethylene (TeflonTM). Recent research et al., 1983a, 1984). at the Waikato Radiocarbon Dating Laboratory has In the next section we summarize how some of these resulted in the development of a new vial made of highimprovements have been achieved. More detailed purity synthetic silica (Hogg et al., 1991). The counting discussions are reviewed by Polach (1987a, b, 1989). characteristics of the new silica vial are compared with We refer specifically to the new technology LS spectro- those of Teflon TM and low-K glass in Table 1. The first meter named the 'Quantulus' that was developed and two columns show background levels and counting assessed by Wallac Oy (Turku, Finland) and Radiocar- efficiencies; the second two columns show the oldest bon Dating Research, Australian National University and youngest ages of samples obtainable using 3 ml (ANU) (Canberra) (Polach et al., 1984, 1988). The first samples of benzene. The results demonstrate that the of three Quantulus LS spectrometers was acquired by synthetic silica vials are as good as those made from the University of Waikato Radiocarbon Dating Labor- TeflonTM, and superior to low-K glass vials, in peratory in 1988. formance. In addition, however, Hogg et al. (1991) found the synthetic silica vials to be the best in terms THE QUANTULUS LIQUID SCINTILLATION of physical and handling properties, making them SPECTROMETER superior even to TeflonTM. Background Reduction by Shielding The Quantulus LS spectrometer achieves ultra-low background levels by both passive and active forms of shielding. The passive shielding consists of a 650 kg asymmetric lead block that encloses the sample photomultiplier tubes (PMTs), and attenuates cosmic and environmental radiation. The shield is constructed from low residual activity lead (from Boliden Mine, Sweden) and the internal cavity surrounding the PMTs is lined with high purity copper (Fig. 1). A layer of cadmium, lined with copper, surrounds the sample PMTs to shield against neutrons (Kojola et al., 1984). Within the lead shield are two sets of PMTs. The lower pair of tubes counts activity in the sample vial, which is hoisted into position. These tubes are totally enclosed by an anti-coincident guard containing a liquid scintillator (Fig. 1). The active, or electronic, shielding consists of the upper pair of PMTs attached to this guard. These tubes can detect and thus reject any pulses caused by cosmic radiation components (such as muons) rather than sample activity (Polach, 1987b).
Data Processing by Spectral Analysis Dual MCAs on the Quantulus allow counting data and instrument performance to be continuously monitored using 'windowless' counting (Polach et al., 1983a). The spectra in Fig. 2 provide an example. They are shown as the logarithm of relative pulse height (energy) of t4c activity, given in terms of MCA channel
TABLE 1, Quantulus counting characteristics* for high-purity synthetic silica 3 ml vials compared with those made from teflon T M and low-K glass Vial material Silica TeflonT M Low-K glass
Background (cpm)
Efficiency (%)
t,,axt (years)
tm~. (years)
0.32 0.29 0.50
83.3 83.7 69.2
55,000 55,400 51,700
40 39 44
*After Hogg et al. (1991). CThe maximum attainable age would increase using vials of larger volume.
New TechnologyLiquid Scintillation Spectrometry
137
more or less evenly around a mean, as they are in Fig. 3.
Electronics and Software The final technical aspect to consider is that the O Quantulus LS spectrometer has been designed with state-of-the-art electronics (Polach, 1987b) that include e.. constant monitoring of the high voltage supply to the PMTs. This is an important development because it provides an extremely stable counting environment, thus improving the accuracy of the instrument for 14C dating. Other electronic controls include a coincidence 26o 560 700 resolution time of 20 nsec, two-position coincidence MCA channel numbers bias, and pulse amplitude comparison circuitory (e.g. FIG. 2. MCA spectra for 1~C sample (S), modern (secondary) see Polach et al., 1983b). Their functions are briefly standard (M), and background(B). Dashed lines L and R flankthe computer-selected 'soft window' best suited for ~4C dating this outlined below. sample (WK-1533a) (see text). (i) Coincidence resolution time. All LS counters using two 180° opposed PMTs record pulses initiated within each PMT only if they fall within a set numbers (0-250 keV) on the horizontal axis, against coincidence resolution time interval. This reduces the counts per channel on the vertical axis. Curve S is that possibility of pulses generated within either of the of the 'unknown' sample being dated; curve M repre- PMTs being recorded as genuine decay events, and sents the higher-activity modern standard; and curve B results in a lowering of background levels. The Quantuis the background activity. In terms of count rates for 5 lus has a very short coincidence time (20 nsec) ml benzene samples, the modern secondary standard compared with the average of 25-40 nsec in conven(ANU Sucrose) is registering 74 counts per minute tional counters. (ii) Two position coincidence bias. In most LS (cpm) (equivalent to 0.95 Oxalic Acid Standard count counters, pulses generated within each PMT are sumrate of 47 cpm), the unknown sample 46 cpm, and the med so long as they occur within the specified coincibackground just 0.4 cpm. Old-technology counters dence resolution time. The Quantulus incorporates a have background count rates of 3-4 cpm, so the two position, factory set coincidence bias threshold that Quantulus backgrounds are around ten times lower the summed pulses must exceed to be recognized as (Polach et al., 1988). true decay events. The coincidence bias threshold is The dashed vertical lines flanking the spectra in Fig. necessary to reduce background due to optical cross2 mark the region of interest or 'soft window' selected talk. Many conventional LS counters utilize a single for counting this particular sample. This window can be coincidence bias level which is optimized for 3H adjusted (unlike old-technology counters in which the window is fixed) to optimize the counting efficiency counting. The two position (HI-LO) switch on the (Polach et al., 1983a). Radioactivity from isotopes Quantulus incorporates coincidence bias levels optiother than 14C, such as radon (222Rn), generate extra mised for both 3H and 14C counting. (iii) Pulse amplitude comparison (PAC). The PAC counts which, if undetected, result in erroneous 14C age determinations (Polach and Kaihola, 1988). The spectral analysis display facility allows such spurious activity to be recognized and hence its influence deleted or 40" corrected. (Radon is generated from traces of uranium 30" and radium present within the sample and can be incorporated into the benzene during synthesis.) 2ff The sample shown in Fig. 2 has an age calculated at o o o 10" around 200 + 20 years after 5000 min (83 hr) of o o o counting time. Counting statistics for this sample (Fig. Mean 3) show how the count rate has varied over this 5000 O min counting period, with each of the points represent-10" ing 500 min of counting time. The distribution of the -20" points therefore provides a measure of stability of the counter - - an unstable counter would show much wider -30" increases or decreases from point to point than the -4~7 pattern shown here. The plot may also show whether Time any short-lived isotope is present as a contaminant in the sample. Thus the data points would tend to lie FIG. 3. Statistical plot of count rate variations over ten 500 min along a negative slope rather than being distributed counting periods. Data are for sample in Fig. 2.
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circuitory compares the amplitude of the pulses from each PMT and, if the pulse heights differ by more than a selected amount, the pulses are rejected as background events. PAC levels are software selectable, and determine the amount of pulse amplitude variation that may be tolerated. The optimum PAC setting is experimentally determined, and results in lower background levels without significant reduction in counting efficiency. Additional software features of the Quantulus include low-level statistical count rate evaluation and validation parameters as well as ~4C age calculations based on the 'windowless' or 'soft window' spectral analysis (Polach, 1989). The spectral analysis facility further improves accuracy through detection of any radioactive contaminants. ADVANTAGES ACHIEVED BY THE QUANTULUS LS SPECTROMETER AND IMPORTANCE TO RADIOCARBON DATING
The advances achieved by the technological improvements described in the previous section arise primarily from the Quantulus' extreme counting stability, its MCA spectral analysis facility, and from the ultra-low backgrounds which reduce standard counting errors. In terms of radiocarbon dating, these advances are advantageous in four main ways. (1) The accuracy of dating is improved. (2) The precision of ages is increased. The best precision achievable by the Quantulus (___20--25 years*) is higher than that of the AMS system (Linick et al., 1989; Gillespie, 1991), and is illustrated in the applications section below. (3) Much smaller samples are required. Although LS spectrometers such as the Quantulus are now capable of dating samples containing 100 mg or less of carbon (Polach, 1987b; minimum amounts depend on sample age), the AMS system is clearly superior in its ability to handle samples with less than 100 mg of carbon (Linick et al., 1989). (4) Older samples can be dated. The maximum theoretical age obtainable using the Quantulus is around 65 ka (Polach, 1987a). Ages exceeding ca. 45 ka generally require large samples which must be unusually well preserved. The previous instrumental age limit using conventional counters was usually in the range 30-40 ka, with AMS having a similar limit (Linick et al., 1989; Gillespie, 1991). APPLICATION TO TEPHRA DEPOSITS IN NEW ZEALAND
In this section of the paper we describe four applications of the Quantulus LS spectrometer to the dating of Late Quaternary tephra deposits in North Island, New Zealand. Three examples relate to tephras *For 5 ml vials of benzene counted for 5000 min.
erupted from the Okataina Volcanic Centre in the Taupo Volcanic Zone (Fig. 4; Froggatt and Lowe, 1990); the fourth example deals with dates on a sequence of tephra layers interbedded with late glacialpostglacial lake sediments. Tarawera Tephra Mt. Tarawera (Fig. 4) last erupted on 10 June, 1886. The eruption was a plinian basaltic event that produced the Tarawera Tephra Formation, devastating much of the surrounding landscape and several villages (Walker et al., 1984; Froggatt and Lowe, 1990). The forest on the slopes of the mountain was killed by the fallout of hot scoria but a number of mature totara trees (Podocarpus hallii) were left standing on the upper slopes (R.F. Keam, pers. commun., 1989) and are still to be found today scattered through the regenerating forest. We estimate that the trees were probably around 200 years old at the time of their death in 1886. They have begun to rot and the outermost wood is commonly soft or has evidently sloughed off. We sampled a branch from one of these trees and dated the most sound outermost wood layers which were extracted with a drill to avoid the obvious borings (Fig. 5a). The four ages obtained (Table 2) range from 180 to 240 BP with standard counting errors of + 30 years. (These and the other ages reported in the text are conventional radiocarbon ages based on the old halflife + 1 standard deviation: Hogg et al., 1987.)The ages on cellulose are likely to be the most reliable because this fraction is the least susceptible to contamination (Gupta and Polach, 1985). The error-weighted pooled mean (Gupta and Polach, 1985) of the four ages is 210 + 15 BP. Recent interlaboratory studies have emphasized the need to use error multipliers in calculating calibrated (calendar) ages using tree ring chronologies because quoted errors may inadequately describe the true variability (e.g. Scott et al., 1990). After applying the 30 year southern hemisphere correction to Stuiver and Pear-
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FIG. 4. Locations of Tarawera and Haroharo volcanoes within the Okataina Volcanic Centre (VC), North Island, and the tephra sites studied: 1, trees killed by fall of Tarawera Tephra (grid reference* V16/194232); 2, Kaharoa Tephra section (V16/175197); 3, Mangaone Tephra section (W16/730378); 4, Lake Maratoto (S15/130663). TVZ = Taupo Volcanic Zone. *Based on the 1:50,000 topographical map series NZMS260.
New Technology Liquid Scintillation Spectrometry
FIG. 5. (a) (upper) Slice from branch of totara tree (Podocarpus hallii) killed by fall of Tarawera Tephra in 1886 (sample WK1534). (Photo: R. Clayton.) (b) (lower) Carbonized logs (Podocarpus hallii) within ignimbrite deposited during Kaharoa eruptive episode. Sample WK-1532 was on bark from the log on the left. Lens cap 5 cm in diameter. (Photo: D.J. Lowe.)
139
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D.J. Lowe and A.G. Hogg
TABLE 2. Radiocarbon ages associated with the tephra deposits examined in this study* and obtained by the Quantulus LS spectrometer Age (BP ± 1 sd:~) Waikato lab no.
Old T1/2
New T1/2
~13C (%o)
Sampler
Tarawera Tephra WK-1533a WK-1533b WK-1534a WK-1534b
210 240 180 210
± ± ± ±
30 30 30 30
210 ± 240± 190± 220 ±
30 30 30 30
-25.2 -25.4 -25.0 -24.0
W CL(1) W CL(2)
620 680 700 660
± ± ± ±
30 35 30 45
640 700 720 680
35 35 30 45
-27.5 -24.7 -24.9 -25.5
CW CW CT CW
-29.1 -29.8
P P
Kaharoa Tephra WK-1531 WK-1532 WK-1535 WK-1346
+ + + +
Mangaone Tephra WK-1525 WK-1526
31,600± 250 31,700± 250
32,500 + 250 32,700 + 250
*Site locations given in Fig. 4. t W = wood; CL(1) = cellulose extracted from WK-1533a; CL(2) = cellulose extracted from WK-1534a; CW = carbonized wood; CT = carbonized twigs; P = Peat. ~sd = standard deviation.
son's (1986) calibration curves, we consequently used an arbitrary theoretical error multiplier of 2.0 to determine the calibrated age for the Tarawera wood. Using 95% confidence limits (two sigma errors), the curves show that multiple calibrated ages are possible, the first having the greatest probability: AD 1722-1820 (58%), AD 1653--1698 (20%), AD 1920-1955 (20%), and AD 1852-1866 (2%). None of these calibrated age ranges encompasses the known year of the Tarawera eruption in AD 1886 but, taking the age of the tree and its condition into account, the most likely age range is still reasonably close. Notwithstanding this, it is evident that Quantulus is capable of precisely dating material as young as 100-200 years. We are currently looking to date more suitable carbonaceous material associated with the emplacement of Tarawera Tephra at other sites. In the interim, results from the Glasgow Intercomparison of Radiocarbon Laboratories Study help to demonstrate the accuracy of the Quantulus for dating very young materials, as attempted for the Tarawera Tephra. Glasgow intercalibration sample 'Wood-3' was dated via dendrochronology at AD 1841-1870 which equates to an expected conventional radiocarbon age of about 100 BP (Aitchison et al., 1990). We obtained a statistically identical age of (WK~cs-1340) 85 + 31 BP for 'Wood-3' using the Quantulus at the Waikato Laboratory.
Kaharoa Tephra The Kaharoa eruptive episode preceded the Tarawera eruption on Mt. Tarawera (Fig. 4). This eruption was rhyolitic and produced airfall tephra deposits, pyroclastic flows and surges, and block-and-ash flows, all of which make up the Kaharoa Tephra Formation (Froggatt and Lowe, 1990). For some time there has
been a question about the age of the eruption, because 14C ages obtained seem to cluster into two groups, one around 900 and the other 700 BP, although there is currently no stratigraphic evidence for an hiatus during the eruptive episode (Froggatt and Lowe, 1990). We obtained four new ages from a range of carbonised materials including twigs, small branches, and bark from small logs (Fig. 5b) in ignimbrite and block-andash flow deposits in a section at the foot of the mountain (Fig. 4). The ages obtained (Table 2) range from 620 to 700 BP with standard counting errors of 30--45 years, and have a pooled mean of 665 + 17 BP. These results thus confirm that the bulk of the Kaharoa deposits were erupted around, or soon after, ca. 700 BP. However, it remains uncertain if a smaller precursory eruptive event took place because these younger ages differ significantly from those of ca. 900 BP. New stratigraphic work and further dating using the Quantulus may resolve this uncertainty.
Mangaone Tephra The Mangaone Subgroup is a group of eight tephra formations of Late Pleistocene age erupted from the Haroharo volcano (Fig. 4; Howorth, 1975). The three youngest formations are, respectively from stratigraphic bottom to top, the Mangaone, Awakeri, and Omataroa tephras. Mangaone Tephra has a pooled mean age (n = 10) of 27,730 + 350 BP; Awakeri Tephra is not dated; and Omataroa Tephra has a pooled mean age (n = 3) of 28,220 + 630 BP (Froggatt and Lowe, 1990). The mean ages for the Omataroa and the Mangaone Tephras are not significantly different yet the Mangaone Tephra must be older than both the Awakeri and Omataroa tephras because it underlies them. Furthermore, the presence of a weak paleosol on the Mangaone Tephra (Howorth, 1975) indicates that some time elapsed before it was buried by the Awakeri and Omataroa deposits. If all the ages on Omataroa Tephra are accepted as valid, then it is evident that some of the dates obtained on the Mangaone Tephra are underestimates. Consequently, Froggatt and Lowe (1990) suggested that Mangaone Tephra was erupted perhaps 30,000 years ago. In order to test this suggestion, we sampled a section at Nukuhou (Site 3 in Fig. 4) containing some Mangaone Subgroup tephra formations interbedded with peat deposits. The section was studied previously by McGlone et al. (1984) who identified the tephra layers present. We confirmed most of the correlations through mineralogy and electron microprobe analysis of glass and dated two 5 cm thick layers of peat at the base of Mangaone Tephra in the upper part of the section. Ages of 31,600 and 31,700 BP, both with standard counting errors of + 250 years (Table 2), were obtained. These results thus support an age of ca. 30,000-32,000 years for the eruption of Mangaone Tephra. In addition, we would like to contrast the relatively low counting errors on these ages with those associated
141
New Technology Liquid Scintillation Spectrometry
with the ages obtained previously on Mangaone Tephra at other locations where the Quantulus was not used. For ten age determinations, these errors range from 400 to 2200 years, but almost all exceed 800 years (see Froggatt and Lowe, 1990), thus demonstrating the improved precision obtainable using the Quantulus LS spectrometer. There are few ages on tephra deposits in New Zealand older than ca. 30 ka (Froggatt and Lowe, 1990) and we are currently attempting to obtain carbonaceous material associated with such undated or poorly dated tephra formations. The oldest ages so far assayed in the Waikato Radiocarbon Dating Laboratory using the Quantulus are > 55,000 years.
unable to date some samples from these lake sediments with low carbon contents because sample activities were statistically indistinguishable from background activities - - the background count rates were too great - - but no such restrictions have occurred using the Quantulus. CONCLUSIONS
New technology LS spectrometry provides both more accurate and more precise dates than conventional instruments, can date much smaller samples than previously possible with radiometric systems, and is capable of extending the limits of detection at both ends of the age scale. These attributes have been Tephra Layers in Lake Sediments achieved in the Quantulus LS spectrometer by providThis final application further emphasizes the value of ing a high degree of counting stability and efficiency in the low background levels attained using the Quantuan ultra low-level background radiation environment lus. Some years ago, Green and Lowe (1985) and Lowe brought about by minimizing all known factors con(1988) took a number of cores from lakes formed ca. tributing to background, and by data evaluation and 17,000 BP near Hamilton in the North Island, including validation through a computer-controlled spectral Lake Maratoto (Site 4 in Fig. 4), as part of a wider analysis facility (Polach, 1987b). The AMS system, pale•environmental project. A large number of the which should be regarded as complementary to LS thin, multiple tephra layers contained within the peaty spectrometry (Polach, 1987a), is capable of dating the sediments were dated using old-style LS counters smallest of samples in a very short measurement time (Hogg et al., 1987). Figure 6 shows a plot of the ages but is less precise and considerably more costly than LS from Lake Maratoto recalculated as if they had been spectrometry. counted using the Quantulus LS spectrometer (normalThe advances provided by the new technology LS ized to 3000 min count time). The horizontal scale gives spectrometry are potentially of great benefit to net count rates from the old LS counter while the tephrochronology and volcanology and to a wide range vertical axis shows differences in standard error beof multidisciplinary applications in Quaternary retween the old LS counter and the Quantulus, expressed search. Four examples relating to tephra deposits in as percentage decrease i.e. (old counter error New Zealand (Tarawera, Kaharoa, and Mangaone Quantulus error/Quantulus error) x 100. For example, tephra formations, and Late Quaternary tephras in lake sample WK-240 was originally dated at 16,900 + 450 sediments) help show these benefits. BP but the equivalent Quantulus date would be 16,900 +_ 135 BP, a reduction in the standard counting error of nearly 250%. ACKNOWLEDGEMENTS Thus, the Quantulus gives greater precision which is of increasing importance as sample activity diminishes We thank Henry Polach and colleagues at the ANU Radiocarbon towards the left hand end of the plot. Furthermore, we Dating Research Laboratory for valued assistance in the operation of have in the past (using old style LS counters) been the Waikato Radiocarbon Dating Laboratory. The continuing support of technical staff at the Waikato Laboratory is also gratefully acknowledged. The University Grants Committee and the University of Waikato Research Committee (UWRC) provided funding towards purchase of two of the Quantulus spectrometers. The UWRC also provided funding for some of the dates reported in this paper. We thank Paul Froggatt (Victoria University of Wellington) for undertaking electron microprobe analyses of tephra samples from the Nukuhou section, Peter de Lange (Department of Conservation) for identifying carbonized wood samples, Ron Keam (University of Auckland) who told us about the in situ trees on Mt. Tarawera killed by the eruption, and an anonymous reviewer who provided useful comments.
250 • WK-240
~ 2oog t50e~ 100-
REFERENCES 50
0
I 2
I 4
I 6
I 8
10
Net count rate (cpm - old counter)
FIG. 6. Normalized net count rates for lake sediment ages (obtained using old-style LS counters, and recalculated as if they had been counted by Quantulus) plotted against differences in standard error expressed as percentage decrease (see text).
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